Structure and orientation of the imidazolium cation at the room-temperature ionic liquid/SiO2 interface measured by sum-frequency vibrational spectroscopy

In this study, we have examined both the effect of alkyl chain length and anion composition on the 1-alkyl-3-methylimidazolium (C(n)mim, n = 4, 6, 8, 10, and 12) structure and orientation at the room-temperature ionic liquid (RTIL)/SiO2 interface by sum-frequency vibrational spectroscopy (SFVS). Four different anions were investigated in this study: tetrafluoroborate (BF4), hexafluorophosphate (PF6), bis(trifluoromethylsulfonyl)imide (BMSI), and bis(pentafluoroethylsulfonyl)imide (BETI). It was found that the alkyl chain in BMSI and BETI RTILs showed a decrease in gauche defects with an increase in chain length, whereas the alkyl chains of the BF4 and PF6 RTILs have virtually no gauche defects regardless of chain length. The tilt of the alkyl chain lies predominantly perpendicular to the surface for all the RTILs examined. A strong correlation between the HCCH v(s) tilt angle and alkyl chain length was observed; as the alkyl chain is lengthened the HCCH v(s) lies more perpendicular to the SiO2 surface. The results of this study suggest that the length of the alkyl chain dictates to a large degree the orientation of the imidazolium cation at the surface, regardless of anion composition. To a lesser extent, the HCCH v(s) tilt of the imidazolium ring of the cation also appears to be correlated to the surface charge density of the SiO2. As the SiO2 surface charge density becomes more negative the HCCH v(s) tilt angle lies more parallel to the surface.